Natural image signals have statistical variations showing nonstationary behavior. One of the entropy coding methods using the nonstationary statistical variations is Context-Based Adaptive Binary Arithmetic Coding (CABAC) (see NPL 1). CABAC is employed as the ITU-T/ISOIEC standard for video coding, H.264/AVC.
The meaning of the terms used in the CABAC scheme will be described hereinafter.
(1) “Context-Based Adaptive” means adapting the coding and decoding methods to the statistical variations. In other words, “Context-Based Adaptive” means predicting an appropriate probability as an occurrence probability of a symbol along with an event of surrounding conditions, when the symbol is coded or decoded. In coding, when an occurrence probability p(x) of each value of a symbol S is determined, a conditional occurrence probability is applied using an actual event or a sequence of events F(z) as a condition.
(2) “Binary” means representation of a symbol using a binary sequence. A symbol represented by a multi-value is once mapped to a binary sequence referred to as “bin string”. A predicted probability (conditional probability) is switched and used for each of the sequence elements, and occurrence of one of the events of the two values is represented by a bit sequence. Accordingly, the probability of a value can be managed (initialized and updated) using a unit (binary element unit) smaller than a unit of a signal type (see FIG. 2 and others of NPL 1).
(3) “Arithmetic” means that the bit sequence is generated not with reference to the correspondences in a table but by the computation. In the coding scheme using the tables of variable-length codes such as H.263, MPEG-4, and H.264, even each value of a symbol with an occurrence probability greater than 0.5 (50%) needs to be associated with one binary sequence (bit sequence). Thus, a value with the greatest probability needs to be associated with one bit for one symbol at minimum. In contrast, the arithmetic coding can represent the occurrence of an event with a higher probability by an integer equal to or smaller than one bit. When (i) there is a signal type in which the occurrence probability of having the first binary value as 0 exceeds 0.9 (90%) and (ii) an event having the first binary value as 0 successively occurs N times, there is no need to output data of 1 bit N times for each value of “0”.